1. Field of the Invention
The present invention relates to a method of manufacturing a rotary screen and specifically to a rotary screen manufacturing method using a galvano process by which a rotary halftone screen with no slit or meshless portion is obtained.
2. Prior Art
FIG. 3(a) shows a conventional rotary halftone screen which is made of nickel and approximately 1 mm in thickness. The rotary halftone screen S has halftone meshes and a linear meshless portion G that is 0.2 to 0.3 mm in width and extends along the entire length of the halftone screen surface.
As shown in FIG. 3(b) and 3(c), the shape of the halftone meshes is, for example, a regular hexagonal (a) or a regular square (b), and the halftone meshes are arranged in regular rows.
When a printing is performed, ink is supplied from the inside of the rotary halftone screen S to the outer surface of the rotary halftone screen by wiping the inside surface of the rotary halftone screen S with an internal doctor so that ink is squeezed out through the meshes. The number of the rows of the halftone meshes is generally 60, 80, 100 or 120 per inch.
In manufacturing rotary halftone screens, there are a lacquer method, a galvano method and an etching method. Among these methods, the galvano method will be described with reference to FIGS. 3(d) through 4(e).
First, as shown in FIG. 3(d), a roll 1, which has surface properties that allow easy stripping of a nickel plating, is coated with a photosensitive film 2. A photo resist film 3 is laid over the photosensitive film 2, and the photosensitive film 2 is exposed by a UV light source 4. As a result, a negative halftone image 5 is made on the roll 1 as shown in FIG. 3(e). Afterward, the roll is plated by nickel so that the nickel 6 is filled in grooves of the halftone image as shown in FIG. 3(f).
The halftone meshes are made by the process described above, and in order to obtain the rotary halftone meshes, following steps are taken.
First, as shown in FIG. 4(a), the roll 1, that is coated with a photosensitive film, is held by a pair of chucks 7 of an exposure apparatus, and one end of the photo resist film 3 is fastened on the surface of the roll 1 by a transparent adhesive tape 8. Afterward, a first masking tape is fastened on the transparent adhesive tape 8 and the roll 1 so that the edge of the first masking tape 9 is accurately aligned with the edge of the photo resist film 3.
Next, as shown in FIG. 4(b), the second stage of the exposure process is performed. After the masking tape 9 is positioned under the UV light source 4, the LrV light source 4 is turned on, and the roll 1 is rotated in the direction indicated by arrow X. An exposure treatment is thus started.
FIG. 4(c) illustrates the third stage of the exposure process. After a prescribed period of time has elapsed, the UV light source is turned off, and a second masking tape 10 is carefully applied so that the edge of the second masking tape 10 is overlapped 0.2 to 0.3 mm from the edge of the masking tape 9.
FIG. 4(d) illustrates the fourth stage of the exposure process. After the second masking tape 10 is applied, the first masking tape 9 is stripped away, and the other end of the photo resist film 3 is fastened by an adhesive tape 11 on the second masking tape 10.
After the photosensitive film is rounded on the entire surface of the roll 1, the UV light source is turned on and the roll is rotated in the direction indicated by arrow X until the second masking tape 10 is positioned under the UV light source 4 as shown in FIG. 4(e). Then, the UV light source is turned off, and the exposure treatment is thus completed.
In the conventional method of manufacturing rotary screens using a galvano process as described above, it is necessary to provide a 0.2 to 0.3 mm overlap of the second masking tape 10 over the first masking tapes 9 so as to make the rotary halftone screen. If the overlap is not provided between the two masking tapes 9 and 10, the both ends of the photo resist film 3 are overlapped as shown in FIG. 5(b). Therefore, a groove on the photosensitive film cannot be formed between the both ends of the photo resist film. In other words, the grooves that are filled with the nickel plating are not formed along the entire length of the roll 1. As a result, rents C are formed in places along the entire length of the halftone screen is not formed as a round shape.
On the other hand, the rotary halftone screen made by the conventional method has a meshless portion that corresponds to the gap as shown in FIG. 5(a). Accordingly, it is impossible to print continuous patterns since ink is not supplied from the meshless portion. If it is desired to print continuous patterns, the holes (a) positioned along the meshless portion is retouched by skilled workers so as to enlarge the holes (a).
Therefore, to make the rotary halftone screen used for continuous printing patterns requires considerable time and labor.